In localized prostate cancer, there is no clear consensus on the treatment alternative that is most advantageous for survival, and each treatment affects health-related quality of life in different ways. In the management of prostate cancer patients, family caregivers often have a major role in shared decision making with the patient and physician. In more advanced stages of the cancer, a family caregiver may become the primary source of information on the health of patient. A crucial issue in HRQL assessment by caregivers is that different perspectives may be elicited from the caregiver, i.e., ask the proxy to respond as they think the patient would [proxy-patient] or from the proxy's own perspective [proxy-proxy], and that each viewpoint can provide important, valid information. However, we are aware of no studies in prostate cancer or otherwise that have compared the HRQL information provided by each of the proxy perspectives, and it is unclear whether they are systematically different. This study proposes to examine whether there is a difference in HRQL assessments made from the proxy-proxy and proxy-patient perspectives in prostate cancer patients, and to compare the extent to which patient self-assessment agrees with each of the proxy perspectives. One hundred prostate cancer patients and their family caregiver proxies will be recruited to complete HRQL assessments of the EORTC QLQ C-30 and EQ-5D using a cross-sectional study design, with the proxy completing 2 assessments (from the proxy-proxy and proxy-patient perspective). By evaluating concordance, statistical significance and clinical meaningfulness of differences between patient and proxy perspectives, the study results will be important in identifying the extent to which the perspective of the proxy influences the information provided by proxy, and will help to inform clinical decision-making and the design and application of standardized proxy-specific HRQL measures in clinical trials for conditions such as prostate cancer where proxy respondents are important.